1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a color filter substrate for a liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes a first substrate and a second substrate that are separated from each other and have a liquid crystal layer interposed therebetween. The first substrate has a common electrode, a black matrix, and a color filter layer, including red, green and blue sub-color filters. The second substrate has a switching element and a pixel electrode.
The black matrix includes a material having an optical density higher than about 3.5. The optical density (OD) is defined by the equation OD=−Log(T), where T is transmittance. For example, a chromium (Cr) thin film or an organic material thin film, including carbon, can be used as the black matrix. In addition, a double layer of chromium and chromium oxide (Cr/CrOx) can be used as the black matrix to reduce reflection from the surface of the double layer.
A photosensitive black resin has been widely used as a black matrix. Since this type of black matrix is formed through coating, exposing and developing steps of the photosensitive black resin, the forming process of this black matrix is simple and fabrication cost is relatively low. A black matrix using a black resin can be used in an in-plane switching mode liquid crystal display (IPS-LCD) device.
FIG. 1 is a schematic cross-sectional view of a substrate for a liquid crystal display device according to the related art. In FIG. 1, a black matrix 5 made of resin is formed on a substrate 3. A color filter layer 7 is formed on the black matrix 5. In general, the black matrix 5 made of resin has an optical density lower than a black matrix made of metallic material. Accordingly, the black matrix 5 made of resin is formed to have a sufficient thickness to prevent light penetration, thereby generating a step in the color filter layer 7. To reduce the effect of the step in the color filter layer 7, an overcoat layer 9 is formed on the color filter layer 7. A common electrode 11 is formed on the overcoat layer 9.
FIGS. 2A to 2E are schematic cross-sectional views showing a fabrication process of a substrate for a liquid crystal display device according to the related art. In FIG. 2A, a black organic layer 13 is formed on a substrate 3 by coating a photosensitive organic material. A mask 15 having a transmissive portion “A” and a blocking portion “B” is disposed over the black organic layer 13. Light is irradiated onto the black organic layer 13 through the mask 15.
In FIG. 2B, a black matrix 5 made of resin is formed on the substrate 3 by developing the irradiated black organic layer 13 (of FIG. 2A). An open portion 10 of the black matrix 5 corresponds to a pixel region. For example, the photosensitive organic material may be a negative type where a non-irradiated potion is removed by developing. In addition, the opening 10 of the black matrix 5 is filled with a color filter layer having red, green and blue sub-color filters.
In FIG. 2C, a red sub-color filter 7a is formed on the black matrix 5 by coating, exposing and developing red resin. Then, a green sub-color filter 7b and a blue sub-color filter 7c are sequentially formed on the black matrix 5 by similar coating, exposing and developing steps. Each sub-color filter 7a, 7b and 7c corresponds to the open portion 10 of the black matrix 5 and overlaps the black matrix 5. Since the black matrix 5 is formed under peripheral portions of each sub-color filter 7a, 7b and 7c, the surface of each sub-color filter 7a, 7b and 7c has a step 20 adjacent to the peripheral portions thereof. For example, each sub-color filter 7a, 7b and 7c may have a step over about 0.8 μm.
In FIG. 2D, an overcoat layer 9 is formed on the color filter layer having red, green and blue sub-color filters 7a, 7b and 7c to reduce the step 20 (of FIG. 2C) of each sub-color filters 7a, 7b and 7c. The overcoat layer 9 includes a transparent insulating resin for planarizing the top surface of the substrate 3.
In FIG. 2E, a common electrode 11 of a transparent conductive material is formed on the overcoat layer 9.
As mentioned above, when a black matrix made of resin is formed on a substrate for an LCD device, a step on a top surface of the substrate is generated due to the thick black matrix. As the surface step increases, the operation of the LCD device becomes unstable. In addition, the surface step causes the appearance of a stain in a displayed image of the LCD device. An overcoat layer is formed to reduce the affects of the step in the surface of the LCD device. However, such an overcoat layer complicates the fabrication process of the LCD device and increases production costs.